The present invention generally relates to exciter circuits used for firing igniter plugs in aircraft and other turbine engine ignition systems.
Gas turbine engine ignition systems typically include an exciter circuit that generates the discharge energy used to fire the igniter plugs (igniters) in the engine. These circuits are commonly used within the aviation industry, but are not limited to aircraft turbine engines. For example, gas turbine generators and other small turbine engines will typically utilize exciter circuits to provide spark energy to one or more igniter plugs. Many exciter circuits in use today utilize a transformer to step-up an input voltage received from a power supply into a controlled succession of high-voltage ionization pulses. The high-voltage nature of these ionization pulses is necessary to successfully cause an ionization current to arc across the air gap between the electrodes of each igniter plug. In this way, each igniter plug is able to properly spark or fire to thereby produce combustion of the fuel in the engine. The specific rate at which an igniter plug is fired in this manner is commonly referred to as the spark rate.
Both inductive and capacitive discharge exciter circuits are known and each generally includes a charging circuit, one or more energy storage devices, and a discharge circuit. For inductive exciter circuits, a transformer is used as the energy storage device, with the energy being stored in a magnetic field produced by driving current through the transformer primary. The energy is periodically delivered to the igniter via the transformer secondary by periodically interrupting the primary current using, for example, a transistor switch in series with the primary, thereby causing transformer flyback which uses the stored magnetic field to generate a high voltage pulse on the secondary.
For capacitive discharge exciters, the energy storage device is a tank capacitor, which is a capacitor having a relatively large charge storage capacity. This capacitor is used to store the spark energy for subsequent delivery to the igniter plug, and can be located at the transformer secondary where it receives high voltage charging current from the transformer. Once the capacitor has been charged up to a predetermined voltage level sufficient for firing the igniter plug, the accumulated charge in the capacitor is discharged through the igniter plug to thereby produce the desired arc for combustion of the fuel. Discharge of the capacitor can be controlled in a known manner using a switch device such as a spark gap within the exciter circuit.
In many capacitive discharge exciter circuits that are powered with an alternating-current (AC) input voltage onboard an aircraft, the charging circuits often include a linear-type transformer in combination with a voltage multiplier for stepping up an input voltage. The voltage multiplier itself is commonly a voltage doubler, such as a cascade-type voltage doubler, that includes two rectifying diodes.
The use of such circuits can be problematic where good regulation of the AC supply is not provided. For example, in aviation applications, the aircraft power system may nominally supply power of 115 VAC at 400 Hertz, with up to about 20% variation in amplitude and frequency being possible. For conventional capacitive discharge exciter circuits that utilize a linear transformer with voltage doubler, these input AC supply variations can result in widely varying input currents and spark rates. Testing of such circuits has shown that the input current can vary from, for example, 0.5 to 5 amps, with the spark rate varying from 1 to 15 sparks per second. Where spark rates of only say 1-3 sparks per second are desired, such higher input currents and spark rates can be undesirable, resulting in reduced operating life of the igniter plugs and perhaps even the exciter circuit itself.
The present invention provides an exciter circuit for firing one or more igniter plugs in an ignition system suitable for operating one or more gas turbine engines on, for example, an aircraft.
In accordance with one aspect of the invention, there is provided an exciter that includes an energy storage device, charging circuit, and discharge circuit. The charging circuit receives input power and stores energy in the energy storage device for subsequent delivery to an igniter via the discharge circuit. The charging circuit has an input, an output, and a ferro-resonant transformer network that includes a saturating transformer with a primary winding connected to the input and a secondary winding connected to the output. Operating power received by the charging circuit via the input is stored in the energy storage device. The discharge circuit has a high voltage output connected to receive spark energy from the energy storage device and this can be by way of a switch device such as a spark gap. The transformer network in the charging circuit includes an inductive coil in series with the primary winding and a capacitor connected to the primary winding. The ferro-resonant transformer network has a resonant frequency that is determined at least in part by the inductance of the coil and the capacitance of the capacitor. The transformer is designed such that, during operation of the exciter circuit, current flow through the coil and the primary causes saturation of the transformer. In this way, the input current used by the exciter can be controlled and limited.
In accordance with another aspect of the invention, there is provided an exciter circuit that includes an input node capable of receiving an alternating-current (AC) voltage with a variable frequency and a variable amplitude, one or more storage devices capable of storing an electric charge, and a charging circuit capable of electrically charging up the one or more storage devices. The charging circuit itself includes a step-up ferro-resonant transformer network and a full-wave rectifier. The step-up ferro-resonant transformer network is electrically connected between the input node and the full-wave rectifier, and the full-wave rectifier is electrically connected between the step-up ferro-resonant transformer network and the one or more storage devices. In addition, the exciter circuit also basically includes a discharge circuit and a switch device. The discharge circuit is electrically connected to the one or more storage devices and is also electrically connectable to the one or more igniter plugs. The switch device is electrically connected to either or both of the one or more storage devices and the discharge circuit. In such a configuration, whenever the one or more storage devices are charged by the charging circuit to a predetermined charge level sufficient for firing the one or more igniter plugs, the switch device is capable of enabling the discharging of the one or more storage devices through the discharge circuit and also the firing of the one or more igniter plugs.
In the disclosed embodiment of the present invention, the step-up ferro-resonant transformer network includes a choke coil, a step-up transformer, and one or more capacitors. The choke coil is preferably a tunable, ferromagnetic-core inductor and has both a first end and a second end. The first end of the choke coil is electrically connected to the input node. The step-up transformer has a ferromagnetic core, a primary winding wrapped about the ferromagnetic core, and a secondary winding wrapped about the ferromagnetic core as well. The primary winding has a first end electrically connected to the second end of the choke coil and a second end electrically connectable to electrical ground. The secondary winding, on the other hand, has a first end and a second end that are both electrically connected to the full-wave rectifier. The one or more capacitors are tapped into the primary winding of the step-up transformer such that the one or more capacitors are electrically connectable between the primary winding and electrical ground. In such a configuration, at least one of the one or more capacitors is preferably a tuning capacitor.
Also, in the disclosed embodiment of the present invention, the discharge circuit includes one or more bleeder resistors and also one or more pulse-forming networks. The one or more bleeder resistors are electrically connected to the one or more storage devices. The one or more pulse-forming networks are electrically connected to the one or more bleeder resistors and are also electrically connectable to the one or more igniter plugs. In such a configuration, each pulse-forming network is preferably an inductor-capacitor peaking network.
Furthermore, in the disclosed embodiment of the present invention, each storage device is a tank capacitor, and the switch device is a spark gap device. The full-wave rectifier is preferably a bridge-type rectifier wherein four rectifying diodes are electrically connected together in a bridge configuration.
When compared with certain conventional capacitive discharge exciter circuits, the exciter circuit disclosed herein has the advantages that it (1) effectively maintains control of the spark rate of the igniter plugs even in aircraft applications wherein the onboard power source supplies an AC input voltage that varies substantially in both frequency and amplitude, (2) takes up less space, (3) is lighter, and (4) is more robust in the event of the failure of a diode in the rectifier section.